If sin x + sin2 x = 1, then write the value of cos8 x + 2 cos6 x + cos4 x.
Given: sin x + sin2x = 1
To find the value of cos8 x + 2 cos6 x + cos4 x.
⇒ sin x = 1 – sin2x
⇒ sin x = cos2x
⇒ cos8x = sin4x, cos6x = sin3x, cos4x = sin2x .
Substituting above values in given equation we get
⇒ sin4x+2 sin3x+ sin2x [(a+b)2 = a2+2ab+b2]
⇒ (sin x + sin2 x)2 = (1)2
⇒ 1